An electron radiographic system, such as is described in U.S. Pat. No. 3,774,029, "Radiographic System with Xerographic Printing", Muntz et al., utilizes fluid absorption of x-rays to produce an electrostatic image on a receptor sheet of dielectric material. The amount of charge collected on the dielectric receptor is proportional to the x-ray dose. Thus the local charge density on a radiograph depends upon the amount of x-ray absorption which occurs along an x-ray path through the patient and thus varies from point to point.
Development of electronradiography images is accomplished by transporting the dielectric sheet with the latent electrostatic image through a toner in a manner similar to that of xerographic office copiers. However, the liquid toners now used for developing electronradiography images are much more sensitive than commercial office copier toners. The sensitivity of a typical electronradiography toner to x-ray dose is shown in curve A of FIG. 3. The high sensitivity of such toners is partially responsible for the high speed of the electronradiography system as compared to standard screen film combinations. However, as shown in curve A, there is no tendency for electronradiography images to saturate at densities above 2.0, as there is for silver halide film.
Although the high contrast of electronradiography images is an asset at densities below 2.0, it presents difficulties at high densities. The contrast achieved at densities above 2.0. is not entirely useful due to limitations in standard lighting procedures (light boxes) and in effect limits the useful latitude of the electronradiography system. In addition, regions of density greater than 4.0 are difficult to dry and fuse during developing. Thus, the ideal system for electronradiography development would maintain the high gamma of the toners for densities below 2.0, but would produce a decreasing gamma as the dose increases to densities above 2.0.
The usefulness of such a saturation phenomenon is demonstrated by considering the optimum densities for diagnosis for various regions of a standard P.A. chest radiograph. These major regions are shown in the table below: Average Areas and Densities for P.A. Chest Surface Optimum Electrostatic Image Area Density Charge Density (cm.sup.2) (average) (n coulomb/cm.sup.2) ______________________________________ Background 100 .about.3.5 .about.33.6 Extraneous 150 2.5 5.82 (Sides) Lung Field 600 1.25 3.82 Heart, Neck 400 .45 2.02 Shoulders Spine, Diaphragm 285 .15 .67 ______________________________________
The high charge density in the background area is due to unattenuated x-rays which have not passed through the patient. This charge density would produce an excessive toner deposit which could spoil the visual image.
Accordingly, it is an object of the present invention to provide new and improved apparatus and method in an electronradiography system for improving the visual image by reducing the charge density in the background area of the latent electrostatic image. A particular object of the invention is to provide for charge density reduction by neutralizing all or a portion of the charge above a predetermined value, so as to achieve a saturation effect.